The element, carbon, is one of the most versatile elements on the periodic table in terms of the number of compounds it may form. It may form virtually an infinite number of compounds. This is largely due to the types of bonds it can form and the number of different elements it can join in bonding. Carbon may form single, double and triple bonds. The hybridization of carbon involved in each of these bonds will be investigated in this handout.

Bonding in any element will take place with only the valence shell electrons. The valence shell electrons are found in the incomplete, outermost shell. By looking at the electron configuration, one is able to identify these valence electrons. Let's look at the electron configuration of ground state (lowest energy state) carbon:

From the ground state electron configuration, one can see that carbon has four valence electrons, two in the 2s subshell and two in the 2p subshell. The 1s electrons are considered to be core electrons and are not available for bonding. There are two unpaired electrons in the 2p subshell, so if carbon were to hybridize from this ground state, it would be able to form at most two bonds. Recall that energy is released when bonds form, so it would be to carbon's benefit to try to maximize the number of bonds it can form. For this reason, carbon will form an excited state by promoting one of its 2s electrons into its empty 2p orbital and hybridize from the excited state. By forming this excited state, carbon will be able to form four bonds. The excited state configuration is shown below:

Since both the 2s and the 2p subshells are half-filled, the excited state is relatively stable.

In order to determine the hybridization on a carbon atom, one must first draw the Lewis structure. From the Lewis structure, count the number of groups around the central carbon. A group represents the regions of electron density around the carbon, and may be single, double or triple bonded. The number of groups represents how many hybrid orbitals have formed. The number of hybrid orbitals formed equals the number of atomic orbitals mixed. The description of the atomic orbitals mixed is equivalent to the hybridization of the carbon atom.

The Lewis structure shows four groups around the carbon atom. This means four hybrid orbitals have formed. In order to form four hybrid orbitals, four atomic orbitals have been mixed. The s orbital and all three p orbitals have been mixed, thus the hybridization is sp3 .

Let's show this using the atomic orbitals of excited state carbon found in the valence shell:

The four sp3 hybrid orbitals will arrange themselves in three dimensional space to get as far apart as possible (to minimize repulsion). The geometry that achieves this is tetrahedral geometry, where any bond angle is 109.5o.

Each hybrid orbital contains 1 electron. A hydrogen 1s orbital will come in and overlap with the hybrid orbital to form a sigma bond (head-on overlap), as shown below:

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Next, let's choose ethene, C2H4 . Draw the Lewis structure:

The Lewis structure show three groups around each carbon atom. This means three hybrid orbitals have formed for each carbon. In order to form three hybrid orbitals, three atomic orbitals have been mixed. The s orbital and two of the p orbitals for each carbon have been mixed, thus the hybridization for each carbon is sp2 .

Let's show this using the atomic orbitals of excited state carbon found in the valence shell:

The three sp2 hybrid orbitals will arrange themselves in three dimensional space to get as far apart as possible. The geometry that achieves this is trigonal planar geometry, where the bond angle between the hybrid orbitals is 120o. The unmixed pure p orbital will be perpendicular to this plane. Keep in mind, each carbon atom is sp2 , and trigonal planar.

Notice that the head-on overlap of sp2 orbitals forms a bond and the side by side overlap of pure p orbitals forms a pi bond between the carbon atoms. This accounts for the carbon-carbon double bond.

The Lewis structure shows two groups around each carbon atom. This means two hybrid orbitals have formed. In order to form two hybrid orbitals, two atomic orbitals have been mixed.

Let's show this using the atomic orbitals of excited state carbon found in the valence shell:

The two sp hybrid orbitals arrange themselves in three dimensional space to get as far apart as possible. The geometry which achieves is linear geometry with a bond angle of 180o. The two pure p orbitals which were not mixed are perpendicular to each other.

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The triple bond consists of one sigma bond and two pi bonds. The geometry around each carbon is linear with a bond angle of 180o.